1. Field of the Invention
The present invention relates to an image display apparatus and a method for driving the apparatus and, more particularly, to a display apparatus having capacitive light-emitting devices, such as organic electroluminescence devices, and the method for driving the apparatus.
2. Description of the Related Art
An electroluminescence display panel which has a plurality of organic electroluminescence devices arranged in a matrix form is receiving great attention as a display which can have lower power consumption and high display quality and can be suitable for thin-profile display apparatus. As shown in FIG. 1, the organic electroluminescence device has at least a single organic function layer 102, comprised of an electron-transport layer, a light-emitting layer, hole-transport layer, etc., and a metal electrode 103, both formed on a transparent substrate 100 like a glass plate on which a transparent electrode 101 is formed. As a positive voltage is applied to the anode of the transparent electrode 101 and a negative voltage to the cathode of the metal electrode 103, i.e., as a DC voltage is applied between the transparent electrode 101 and the metal electrode 103, the organic function layer 102 emits light. With the organic function layer formed of an organic compound which can be expected to have an excellent emission characteristic, the electroluminescence display can be used practically.
An organic electroluminescence device (hereinafter also referred to as "EL device") can be expressed as an electrically equivalent circuit as shown in FIG. 2. As apparent from the circuit diagram, the device can be replaced with a capacitive component C and a component E with a diode characteristic that is coupled in parallel to the capacitive component C. The EL device is thus a capacitive light-emitting device. When a DC drive voltage is applied between the electrodes of the EL device, charges are stored in the capacitive component C. When the drive voltage exceeds the barrier voltage or emission threshold value inherent to the device, a current starts flowing into the organic function layer that has the light-emitting layer from one of the electrodes (the anode side of the diode component E) and light is emitted with the intensity proportional to the current.
The voltage V v.s. current I v.s. luminance L characteristic of the device is similar to the diode characteristic such that the current I is very small for the voltage equal to or lower than the emission threshold value Vth but abruptly increases when the voltage becomes greater than the emission threshold value Vth, as shown in FIG. 3. The current I is approximately proportional to the luminance L. Such a device provides a luminance proportional to the current that accords to the drive voltage when the drive voltage above the emission threshold value Vth is applied to the device, but it has substantially no drive current flowing when the applied drive voltage is lower than the emission threshold value Vth, so that the luminance stays substantially equal to zero.
Passive matrix driving can be used to drive a display panel which uses a plurality of such EL devices. FIG. 4 exemplifies the structure of a passive matrix display panel. An N number of cathode lines (metal electrodes) B.sub.1 to B.sub.n are laid horizontally, and an M number of anode lines (transparent electrodes) A.sub.1 to A.sub.m are laid in parallel vertically to cathode lines B.sub.1 -B.sub.n, with light-emitting layers of EL devices E.sub.1,1 to E.sub.m,n placed at (a total of n.times.m) intersections between the anode lines A.sub.1 -A.sub.m and the cathode lines B.sub.1 -B.sub.n. The devices E.sub.1,1 to E.sub.m,n which serve as pixels are arranged in a grid pattern, and have their one ends (each of which corresponds to the anode of the diode component E in the aforementioned equivalent circuit) connected to the anode lines A.sub.1 -A.sub.m at the respective intersections between the vertical anode lines A.sub.1 -A.sub.m and the horizontal cathode lines B.sub.1 -B.sub.n and the other ends (each of which corresponds to the cathode of the diode component E in the equivalent circuit) connected to the cathode lines B.sub.1 -B.sub.n. The cathode lines B.sub.1 -B.sub.n are connected to, and driven by, a cathode-line scan circuit 1, while the anode lines A.sub.1 -A.sub.m are connected to, and driven by, an anode-line driver 2.
The cathode-line scan circuit 1 has scan switches 5.sub.1 to 5.sub.n which are associated with the cathode lines B.sub.1 -B.sub.n and respectively determine the potentials of the cathode lines B.sub.1 -B.sub.n. Each of the scan switches 5.sub.1 -5.sub.n connects either a reverse bias voltage V.sub.CC (e.g., 10 V), which is a power supply voltage, or a ground potential (0 V) to the associated cathode line.
The anode-line driver 2 has current sources (e.g., constant current sources) 2.sub.1 to 2.sub.m and drive switches 6.sub.1 to 6.sub.m, which are associated with the anode lines A.sub.1 -A.sub.m and supply the drive current to the respective devices via the respective anode lines. The anode-line driver 2 performs ON/OFF control on the drive switches 6.sub.1 -6.sub.m to let the current flow through the respective anode lines A.sub.1 -A.sub.m individually. It is typical to use current sources as the drive sources instead of voltage sources like constant voltage sources for reasons such as the aforementioned current v.s. luminance characteristic being stable with respect to a temperature variation whereas the voltage v.s. luminance characteristic is not. The amount of the current to be supplied from each of the current sources 2.sub.1 -2.sub.m is set to the amount that is necessary to keep the associated device emitting light at the desired instantaneous luminance (hereinafter this state will be called "steady emission state"). As electrical charges are being stored in the capacitive component C in the device while the device is in the steady emission state, the voltage across the device becomes a specified value Ve (hereinafter called "specified emission voltage").
The anode lines A.sub.1 -A.sub.m are also connected to an anode-line resetting circuit 3, which has shunt switches 7.sub.1 -7.sub.m provided for the respective anode lines. As each shunt switch is selected, the anode-line resetting circuit 3 sets the associated anode line to the ground potential.
The cathode-line scan circuit 1, the anode-line driver 2 and the anode-line resetting circuit 3 are connected to an emission controller 4.
In accordance with image data supplied from an image data generating system (not shown), the emission controller 4 controls the cathode-line scan circuit 1, the anode-line driver 2 and the anode-line resetting circuit 3 to display images carried by the image data. The emission controller 4 controls switching of the scan switches 5.sub.1 -5.sub.n to send a scan-line selection control signal to the cathode-line scan circuit 1, select one of the cathode lines that corresponds to the horizontal scan period of the image data, connect the selected cathode line to the ground and apply the reverse bias voltage V.sub.CC to the other cathode lines. The reverse bias voltage V.sub.CC is applied by a constant voltage source to be connected to each cathode line in order to prevent cross-talk emission from the devices connected at the intersections of the driven anode lines and the cathode lines which are not selected for scanning. The reverse bias voltage V.sub.CC is generally set equal to the specified emission voltage Ve. As the scan switches 5.sub.1 -5.sub.n are sequentially switched to the ground potential every horizontal scan period, the cathode line which has been switched to the ground potential serves as a scan line which permits the devices connected to the cathode line to emit light.
The anode-line driver 2 performs drive control on the selected scan line. The emission controller 4 generates drive control signals (drive pulses) indicating which device connected to the scan line should be enabled to emit light at what timing and for how long, in accordance with pixel information specified by the image data, and sends the drive control signal to the anode-line driver 2. In accordance with the drive control signal, the anode-line driver 2 implements ON/OFF control on some of the drive switches 6.sub.1 -6.sub.m and supplies the drive current to the devices corresponding to the pixel information via the associated anode lines A.sub.1 -A.sub.m. Consequently, the devices supplied with the drive current emit light according to the pixel information.
The reset operation of the anode-line resetting circuit 3 is performed in response to a reset control signal from the emission controller 4. The anode-line resetting circuit 3 sets any of the shunt switches 7.sub.1 -7.sub.m which corresponds to the anode line to be reset that is indicated by the reset control signal, and sets off the other shunt switches.
Japanese Laid-Open Patent Publication (KOKAI) No. H 9-232074 of the same applicant as the present application discloses a driving method of executing a reset operation to discharge electrical charges stored in individual devices laid out in a grid pattern on a passive matrix display panel immediately before changing the scan line (this method will be hereinafter called "reset driving method"). The reset driving method quickens the rising of emission of devices at the time the scan line is changed over to another one. The reset driving method for a passive matrix display panel will now be described with reference to FIGS. 4 to 6.
The operation exemplified in FIGS. 4 to 6 is for a case where the cathode line B.sub.1 is scanned to permit the devices E.sub.1,1 and E.sub.2,1 to emit light, then scanning is shifted to the cathode line B.sub.2 to permit the devices E.sub.2,2 and E.sub.3,2 to emit light. For easier understanding of the description, the devices which are emitting light are indicated by the symbols of diodes, while the devices which are not emitting light are indicated by the symbols of capacitors. The reverse bias voltage V.sub.CC to be applied to the cathode lines B.sub.1 -B.sub.n is 10 V, the same as the specified emission voltage Ve for the devices.
First, only the scan switch 5.sub.1 is switched to the ground potential position and the cathode line B.sub.1 is scanned in FIG. 4. The reverse bias voltage V.sub.CC is applied to the other cathode lines B.sub.2 -B.sub.n by the scan switches 5.sub.2 -5.sub.n. At the same time, the current sources 2.sub.1 and 2.sub.2 are respectively connected to the anode lines A.sub.1 and A.sub.2 by the drive switches 6.sub.1 and 6.sub.2. The other anode lines A.sub.3 -A.sub.m are switched to the ground potential (earth) position of 0 V by the shunt switches 7.sub.3 -7.sub.m. In the case of FIG. 4, therefore, only the devices E.sub.1,1 and E.sub.2,1 are biased in the forward direction, and the drive current flows into those devices from the current sources 2.sub.1 and 2.sub.2 as shown by the arrows, causing only the devices E.sub.1,1 and E.sub.2,1 to emit light. In this state, the devices E.sub.3,2 to E.sub.m,n which are not emitting light and are indicated by hatching are charged to the illustrated polarity.
The following reset control is executed immediately before scanning is shifted from the steady emission state in FIG. 4 to a state where the next devices E.sub.2,2 and E.sub.3,2 emit light. Specifically, as shown in FIG. 5, all the drive switches 6.sub.1 -6.sub.m and scan switches 5.sub.1 -5.sub.n are connected to the voltage V.sub.CC and all the shunt switches 7.sub.1 -7.sub.m are opened. When the all-resetting is carried out, all of the anode lines and the cathode lines have the same potential, so that the charges stored in the individual devices are discharged through the routes indicated by the arrows in FIG. 5. As a result, the charges stored in all the devices will vanish instantaneously.
After the charges stored in all the devices are set to zero, only the scan switch 5.sub.2 corresponding to the cathode line B.sub.2 is switched to the 0 V position to scan the cathode line B.sub.2 as shown in FIG. 6. At the same time, the drive switches 6.sub.2 and 6.sub.3 are closed to connect the current sources 2.sub.2 and 2.sub.3 to the associated anode lines, and the shunt switches 7.sub.1 and 7.sub.4 -7.sub.m are switched on to apply 0 V to the anode lines A.sub.1 and A.sub.4 -A.sub.m.
As apparent from the above, the emission control in the reset driving method repeats the scan mode during which one of the cathode lines B.sub.1 -B.sub.n is set active and the following reset mode. The scan mode and reset mode are performed every horizontal scan period (1 H) of image data. If the state in FIG. 4 were shifted to the state in FIG. 6 directly without the reset control, the drive current to be supplied from the current source 2.sub.3, for example, not only would flow into the device E.sub.3,2 but would also be used to cancel the charges of the opposite polarity (shown in FIG. 4) stored in the devices E.sub.3,3 to E.sub.3,n. It would therefore take time to render the device E.sub.3,2 in the steady emission state (to set the voltage across the device E.sub.3,2 to the specified emission voltage Ve).
Through the above-described reset control, however, the potentials of the anode lines A.sub.2 and A.sub.3 become approximately V.sub.CC the instant scanning is shifted to the cathode line B.sub.2, so that the charge current flow into the devices E.sub.2,2 and E.sub.3,2 which should emit light next, through a plurality of routes from the constant voltage sources connected to the cathode lines B.sub.1 and B.sub.3 -B.sub.n as well as from the current sources 2.sub.2 and 2.sub.3. The charge current make the voltages across the devices E.sub.2,2 and E.sub.3,2 reach the specified emission voltage Ve instantaneously, thus enabling instantaneous transition to the steady emission state.
Since the conventional reset driving method temporarily resets all of the cathode lines and the anode lines by connecting those lines to the ground potential of 0 V or the same potential as the reverse bias voltage V.sub.CC before emission control moves to the next scan line, it is possible to speed up charging of the devices to emit light in the next scan to the emission threshold value Vth at the time the scan line is switched and quicken the rising of emission of the devices on the switched scan line which should emit light.
The charges stored in parallel capacitive components of the devices that are to emit light are discharged before starting each scanning in the passive matrix display panel employing the aforementioned reset driving method. Thus, it has a deficiency that the electrical energy is disadvantageously wasted, particularly when displaying images with low lighting ratio. Paying attention to a case where the EL devices E.sub.m,1 and E.sub.m,2 connected to the anode line A.sub.m do not emit light when the scanning target is switched from the cathode line B.sub.1 to the cathode line B.sub.2 as shown in FIGS. 4 to 6, for example, the power loss of those devices will be explained referring to FIGS. 7A through 7C. As shown in FIG. 7A, while the device E.sub.m,1 is not charged during the first scanning of the cathode line B.sub.1 due to the cathode line B.sub.1 and anode line A.sub.m both being at the ground potential, the devices E.sub.m,2 to E.sub.m,n are biased in the reverse direction with the reverse bias voltage V.sub.CC and their parallel capacitive components are charged with charges Q via the cathode lines B.sub.2 -B.sub.n. The total amount of charges of the devices on the anode line A.sub.m which are not emitting light becomes (n-1)Q. Next, all-resetting to V.sub.cc causes all the charges (n-1)Q to be discharged via the anode line A.sub.m and cathode lines B.sub.2 -B.sub.n, and the charge of the device becomes zero, as shown in FIG. 7B. During the second scanning of the next cathode line B.sub.2, as shown in FIG. 7C, each of the parallel capacitive components of the devices E.sub.m,1 and E.sub.m,2 to E.sub.m,n on the anode line A.sub.m are charged with charges (n-1)Q. When one pays attention to the devices which do not emit light, therefore, wasteful discharging occurs every resetting operation. In other words, in a case where an anode line is reset between the first and second scans and the devices on that anode line, such as the devices E.sub.2,1 and E.sub.2,2 on the anode line A.sub.2, are rendered off from off, consumed power of charges 2(n-1)Q is wasted. The power loss by the charging and discharging of the parallel capacitive components in a plurality of EL devices of the display panel becomes greater in proportion to the parallel capacitance per unit area and the effective area of the display panel. It is therefore necessary to reduce the power loss.